1. Technical Field
This invention relates generally to rechargeable electrochemical battery cells, and more particularly to impact resistant packaging for such cells.
2. Background Art
Portable, battery-operated, electronic devices seem to be everywhere. From handheld games, to compact disc players, to radios, to personal data assistants (PDAs), to phones, to pagers, it is becoming rare to encounter a person who does not carry at least one portable electronic device with them all the time. People carry the devices for entertainment, for organizational purposes, and for staying connected with others. A common characteristic shared by each of these devices is that they all rely on batteries for portability.
Batteries are manufactured by taking two electrically opposite electrodes and stacking them together, with each electrode being physically separate from the other. A common way to manufacture the electrochemical cells used in the batteries is known as the xe2x80x9cjellyrollxe2x80x9d technique, where the inner parts of the cell are rolled up and placed inside an aluminum can, thereby resembling an old-fashioned jellyroll cake. Aluminum is the preferred metal for the can due to its light weight and favorable thermal properties. To understand the jellyroll technique, consider the following example:
Cells are made of a positive electrode (cathode), a negative electrode (anode), and a separator that prevents these two from touching, while allowing electrons to pass through. Referring now to FIG. 1, illustrated therein is a cross-sectional side view of a typical electrode layer assembly. The electrode 10 includes a separator 12 having a top and bottom 14 and 16. Disposed on the top 14 of the separator 12 is a first layer 18 of an electrochemically active material. For example, in a nickel metal hydride battery, layer 18 may be a layer of a metal hydride charge storage material as is known in the art. Alternatively, layer 18 may be a lithium or a lithium intercalation material as is commonly employed in lithium batteries.
Disposed atop layer 18, is a current collecting layer 20. The current collecting layer may be fabricated of any of a number of metals known in the art. Examples of such metals include, for example, nickel, copper, stainless steel, silver, and titanium. Disposed atop the current collection layer 20 is a second layer 22 of electrochemically active material.
Referring now to FIGS. 2 and 3, illustrated therein is stack of electrodes like that in FIG. 1 assembled in the jellyroll configuration so as to make a rechargeable cell. In FIGS. 2 and 3, two electrodes 40 and 60 are provided as described above. Electrode 40 is fabricated with two layers of, for example, negative/active electrochemical material while electrode 60 is fabricated with two layers of positive electrode material. Each electrode 40,60 is provided with a current collecting region 20. The current collecting region 20 is disposed on the current collector, and allows for electrical communication between the electrode itself and a terminal on the outside of the cell can into which the electrode stack of FIG. 2 may be inserted. While the current collecting region 20 is disposed on the top and bottom of the jellyroll in this exemplary embodiment, note that they may equally be located at the leading and trailing edges of the jellyroll as well.
The electrodes 40 and 60 are arranged in stacked relationship with the current collecting regions 20 disposed on opposite edges of the stack. Thereafter, the stack is rolled into a roll 70 for a subsequent insertion into an electrochemical cell can. The cans are generally oval, rectangular or circular in cross section with a single opening and a lid. This is similar to the common trashcan.
Referring now to FIG. 3, illustrated therein is a cross-sectional cut-away view of the stacked configuration shown in FIG. 2. Here, electrodes 40 and 60 can be seen in stacked orientation. Electrode 40 comprises substrate 42 first layer of negative active material 44, current collecting layer 46, and second layer of active material 48. Disposed immediately atop layer 48 is the separator 62 of electrode 60. Thereafter the first layer of active material 64 is disposed atop the separator 62 with current collecting layer 66 disposed there over and second layer of active material 68 disposed atop the current collecting layer.
As the configuration is rolled into roll 70, the outer membrane layer is rolled into contact with the membrane substrate layer 42 of electrode 40 is rolled into contact with the second layer of active material 68 of electrode 60. In this way, the membrane substrate layers act as a separator to electrically isolate the positive and negative electrodes from one another. Moreover, as the membranes are porous, they may be filled with a liquid electrolyte such as is known in the art. Accordingly, the membrane allows for deposition of ultra-thin electrode layers, and current collecting layers, while providing the function of both electrolyte reservoir and separator. The result is ultra-thin electrodes having extremely high capacity.
Once the jellyroll is complete, it is inserted into a metal can 122 as shown in FIG. 4. The metal can 122 includes a first metal connector 24 that may serve as the cathode and a second metal connector 26 capable of serving as the anode. Looking to the jellyroll, the various layers can be seen: separator 34, first electrode 34, and second electrode 36. Depending upon the construction, an electron or current collector or grid 38 may be added to the device if desired. The current collector 38 is typically formed from a metal such as cobalt, copper, gold, iron, manganese, nickel, platinum, silver, tantalum, titanium, or zinc.
Traditionally, such metal-can type batteries were inserted into plastic battery housings that included circuitry like protection circuits, charging circuits, fuel gauging circuits and the like. The plastic battery housings were then used with electronic host devices. However, as electronic devices have gotten smaller and smaller, manufacturers have begun putting the associated battery circuitry in the host device. Thus, they use just the metal-can battery, without protective plastic housing, in their devices.
This creates a problem in that, as stated above, the metal cans are generally made from soft metals like aluminum. Thus, when the metal-can battery is dropped, the can may dent, bend and deform. Recall from above that it is important in battery construction that the cathode and anode be kept apart by the separator or membrane layer. If the metal can bends or dents, this may cause the cathode and anode to touch inside the can, thereby creating a short circuit condition in the can. Short circuit conditions can lead to high currents that generate high temperatures and seriously compromise reliability of the battery.
There is thus a need for an improved metal-can battery assembly that prevents short circuit conditions caused by impact related deformations in the metal can.
FIG. 1 is a cross-sectional side view of a typical prior art electrode layer assembly.
FIGS. 2 is a prior art stack of electrodes assembled in the jellyroll configuration so as to make a rechargeable cell.
FIG. 3 is a cross-sectional cut-away view of the stacked configuration shown in FIG. 2.
FIG. 4 is cut away, cross sectional view of a prior art jellyroll inserted into a metal can.
FIG. 5 is a cross sectional view of a prior art metal-can battery that has been repeatedly dropped on a hard surface as is typical in OEM quality and qualification practice.
FIG. 6 is a cell assembly in accordance with the invention.
FIG. 7 is a comparison of cross-sectional views of the prior art cell and a cell in accordance with the invention.